The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The invention is related in general to wellsite equipment such as wireline surface equipment, wireline cables and the like.
Deviated wells or wellbores often include extensive horizontal sections in additional to vertical sections. During oilfield operations, it can be particularly difficult to advance tool strings and cables along these horizontal sections. While tool strings descend by gravity in vertical well sections, tractor devices, which are attached to the tool strings are used to perform this task in the horizontal sections, such as those shown in FIG. 1.
In particular, FIG. 1 illustrates a downhole tractor assembly 100 including a tractor 102 coupled to a tool string 104 and a cable 106 coupled to the tool sting 104 opposite the tractor 102. In operation, the tractor 102 pulls the tool string 104 and the cable 106 along a horizontal well section, while a swivel connection 108 coupled between the tool string 104 and the cable 106 minimizes a rotation of the cable caused by a rotation of the tractor 102 and tool string 104.
Several problems are associated with tractor or tractoring operations including torque imbalances in wireline cables that may lead to knotting or bird caging during sudden releases of cable tension. Uneven surfaces of wireline cables can abrade or saw into bends in well casings, which may damage the cable and well casing or cause the cable to become stuck.
A weight of the wireline cables imparts a drag on the tractor and the associated equipments such as a tool string and the like. The speed of travel of the tractor, therefore, is limited by the cable weight. The longer and/or more deviated the well, the more power the tractor requires in order to pull the weight of the cable and associated equipment.
A typical wireline cable with metallic armor wires on the outside diameter thereof has high friction with the wellbore including the casing and the like. Much of the power of the tractor, therefore, is used to overcome the friction between the cable and the wellbore. Due to the high friction between the cable and the wellbore a greater pulling power at the surface is also needed in the event of a tractor failure, wherein the cable is used as a life line to pull the tractor assembly out of the well.
Typical wireline cables have about 98% coverage in their outer armor wire strength member layer to fill the armor wire layer to be able to handle the cable and provide protection for the cable core. Due to this coverage, torque imbalances are inherent in this type of wireline cable, which may cause the cable to rotate during changes in the cable tension.
As the tractor travels down the well it may take a tortuous path and that can rotate the cable. To avoid rotating the cable, a swivel connection is used to connect the cable to the tool string to isolate the tool string from this type of torque. Because torque is generated in the cable when under tension, during a sudden release of that tension, the swivel allows the cable to spin, which can result in opening up of the outer armor wires (i.e. birdcaging) and may disadvantageously cause the cable to loop over itself within the casing.
Mono-cables with alloy armor wires typically comprise a single insulated copper conductor at the core for both electrical transmission and telemetry functions. With mono-cables, electric power is transmitted down the central, insulated power conductor and the electric power returns along the armor. However, with long length alloy cables, electrical power return on them is not possible as a galvanized steel armor package is utilized and the highly resistive nature of alloy wires, such as MP35N and HC-265, effectively precludes the production of long length mono-cables with alloy armors. In order to overcome the above issue, coaxial cables were introduced. With coaxial cables, the electrical power is transmitted down a central, insulated conductor, and returns along a serve layer of stranded copper wires covered by a thin layer of polymeric insulation located near the outer edge of the cable core. However, both mono-cables and coaxial cables have the same disadvantages during tractoring operations, as disclosed above.
It remains desirable to provide improvements in wireline cables and/or downhole assemblies. It is desirable, therefore, to provide a cable that overcomes the problems encountered with current cable designs.